The present invention relates to a spread-spectrum demodulator in radio communication, which receives a spread signal transmitted upon frequency spreading of a desired signal by computation using a spreading code, and extracts the desired signal by despreading the received spread signal by computation using the spreading code.
FIG. 38 shows the arrangement of a spread-spectrum demodulator according to the first prior art. In this arrangement, a multiplier 2002 multiplies a received spread signal by the spreading code generated by a spreading code generating circuit 2001. An output from the multiplier 2002 is filtered by a bandpass filter 2005 to extract only a signal component in a necessary band, and an amplitude detector 2008 detects it. A spreading code leading in phase with respect to the spreading code supplied to the multiplier 2002 is supplied to a multiplier 2003. The multiplier 2003 multiplies this spreading code and the spread signal. A spreading code lagging in phase with respect to the spreading code supplied to the multiplier 2002 is supplied to a multiplier 2004. The multiplier 2004 multiplies this spreading code and the spread signal. An output from the multiplier 2003 is filtered by a bandpass filter 2006 and detected by an amplitude detector 2009. Likewise, an output from the multiplier 2004 is filtered by a bandpass filter 2007 and detected by an amplitude detector 2010. The passbands of the bandpass filters 2005 to 2007 are almost the same as the band of data signals. A subtracter 2011 computes the difference between an output from the amplitude detector 2009 and an output from the amplitude detector 2010. A multiplier 2012 multiplies an output from the amplitude detector 2008 and an output from the subtracter 2011. A loop filter 2013 integrates an output from the multiplier 2012 to generate a control voltage. A voltage-controlled oscillator 2014 supplies a clock having frequency proportional to the control voltage to the spreading code generating circuit 2001.
If no synchronization can be established between a spread signal and a spreading code, low-power, noise-like signals are output from the bandpass filters 2005 to 2007. When the spread signal slightly leads in phase in a synchronized state, a large signal appears in the bandpass filter 2006, and a large detection output is obtained from the amplitude detector 2009. When the spread signal slightly lags in phase in a synchronized state, a large output is obtained from the bandpass filter 2007. In the synchronized state, a large output signal is obtained from the amplitude detector 2008. According to the arrangement in FIG. 38, clocks to be supplied to the spreading code generating circuit 2001 are controlled with high precision by using outputs from the three amplitude detectors 2008 to 2010, thereby obtaining a data signal from the amplitude detector 2008.
FIG. 39 shows the arrangement of a spread-spectrum demodulator according to the second prior art. In this arrangement, a matched filter 2111 corresponding to a spreading code converts a received spread signal into a correlation signal, and a delay line 2112 delays the correlation signal by the reciprocal of a data clock. A multiplier 2113 multiplies the delay signal and the correlation signal. A peak detector 2114 then detects the peak of the multiplication result to obtain a data signal. FIG. 40A shows the waveform of an output from the multiplier 2113 in the spread-spectrum demodulator in FIG. 39. FIG. 40B shows the waveform of an output from the peak detector 2114.
The spread-spectrum demodulator having the synchronous control circuit in FIG. 38 and the spread-spectrum demodulator having the matched filter in FIG. 39 are disclosed in, for example, Gen Marubayashi, Masao Nakagawa, and Ryuji Kohno, “Spread Spectrum Communication and Its Applications”, IEICE, 1998, pp. 94-145, ISBN4-88562-163-X”.
In the spread-spectrum demodulator as the first prior art shown in FIG. 38, a spreading code and a spread signal must be set in phase with each other with high precision. This complicates the circuit arrangement and increases the circuit size and power consumption.
In the spread-spectrum demodulator as the second prior art shown in FIG. 39, a general SAW (Surface Acoustic Wave) filter is used as the matched filter 2111. This leads to increases in implementation area and implementation cost. In addition, since the matched filter 2111 specialized for a specific spreading code is used, a spread signal with a different spreading code cannot be demodulated. In addition, if the matched filter 2111 is formed from an on-chip circuit, the area power consumption increase.